Pressure casting process

ABSTRACT

A PRESSURE CASTING APPARATUS FOR CASTING HIGHLY VISCOUS MATERIAL HAVING A CASTING MOLD WITHIN A VACUUM BELL, WHEREBY THE VACUUM PREVENTS VOIDS IN THE CAST MATERIAL AND REDUCES THE PRESSURIZATION OTHERWISE REQUIRED ON A TRANSFER POT CONTAINING THE MATERIAL TO BE CAST. THE TRANSFER POT IS OPERABLY CONNECTED TO A PRESSURE CASTING HEAD HAVING PATTERN HOLES IN THE BOTTOM THEREOF, WHICH CASTING HEAD FORMS THE TOP OF THE VACUUM BELL ABOVE THE CASTING MOLD. THE CASTING PROCESS IS PERFORMED BY DRAWING A VACUUM IN THE BELL, PRESSURIZING THE TRANSFER POT, FORCING THE MATERIAL FROM THE TRANSFER POT THROUGH THE CASTING HEAD, AND DROPPING THE EXTRUDED MATERIAL INTO THE CASTING MOLD FOR CURING.

Aug. 17,

PRESSURE CASTING PROCESS Filed Dec. 4, 1967 PRESSURE PUMP ssw P 0 0 3 20 k FL I 24 Rae 15H /z INVENTOR.

LLOYD A. WALKER RUDOLPH A. PETERSON M -M 7% m. aw-

ATTORNEYS 3,600,486 PRESSURE CASTING PROCESS Lloyd A. Walker, Placerville, and Rudolph A. Peterson, Rancho Cordova, Calif., assignors to Aerojet-General Corporation, El Monte, Calif.

Filed Dec. 4, 1967, Ser. No. 687,651 Int. Cl. 132% /00 US. Cl. 264-89 5 Claims ABSTRACT OF THE DISCLOSURE A pressure casting apparatus for casting highly viscous material having a casting mold within a vacuum bell, whereby the vacuum prevents voids in the cast material and reduces the pressurization otherwise required on a transfer pot containing the material to be cast. The transfer pot is operably connected to a pressure casting head having pattern holes in the bottom thereof, which casting head forms the top of the vacum bell above the casting mold. The casting process is performed by drawing a vacuum in the bell, pressurizing the transfer pot, forcing the material from the transfer pot through the casting head, and dropping the extruded material into the casting mold for curing.

BACKGROUND OF THE INVENTION The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Army.

This invention relates to a new and useful pressure casting apparatus for form casting highly viscous material into desired and useable configurations and the process for performing the casting operation by using this apparatus. This invention is particularly useful in the form casting of solid rocket propellants whereby a highly viscous propellant mixture is cast into a desired form and cured, after which it is further processed for utilization in a solid rocket motor.

Prior to this invention highly viscous material could be molded or form cast into desired configurations. One method of molding the material was to pour the highly viscous material into a mold using care to prevent voids within the mold. However, by this method, components within the mixture, such as metal staples, are randomly disposed throughout the mold. Another method used prior to this invention was the use of a vacuum bell with a screen therein. A charge of highly viscous material would be placed upon the screen and enclosed in the upper portion of the bell. A vacuum was drawn in the lower portion of the vacum bell and a slight vacuum in the upper portion of the bell. The differential in pressure across the screen would cause the viscous material to be drawn through the screen as extruded fascicles. The vacuum hell would then be vibrated to control the length of the fascicles. The upper portion of the bell above the screen had to be recharged prior to each molding operation. This required disassembly of the vacuum bell and filling the upper portion thereof with suflicient viscous material so that openings would not develop through the material to destroy the differential in vacuum pressures. Each charge for the casting operation had to be substantially more material than would be required to fill the mold.

SUMMARY OF THE INVENTION In the instant invention, a casting head defines a cavity through which highly viscous material is forced. The casting head rests on a vacuum bell within which there is a casting mold, and the casting head is of sufficient height above the mold so that the viscous material stretches and breaks prior to being deposited into the mold. The casting head is operably connected to a transfer pot containing the viscous material to be cast. Within the connection between the transfer pot and the casting head there is arranged a flexible coupling which allows the casting head to be lifted from the vacuum bell so that a filled mold may be removed and a fresh mold inserted. Several casting heads may be manifolded to a single transfer pot so that several casting operations may be simultaneously in process. A predetermined amount of highly viscous material is charged into the transfer pot. A vacuum is drawn on the vacuum bell and the transfer pot is pressurized with a gas compatible with the viscous material. The rate of flow through the casting head and the length of fascicles extruded through the casting head are a function of the difference in pressure between the transfer pot and the vacuum bell. The fascicles as they are extruded through the casting head fall into the mold, and as they are elongated fascicles, they fall into a horizontal plane so that any components, such as metal staples, contained in the fascicle become oriented in the horizontal plane, although they may be randomly dispersed within that plane. When the mold is full, the vacuum on the vacuum bell is released and the pressure on the transfer pot is reduced to prevent flow of viscous material through the casting head. The casting head may then be removed from the vacuum bell so that a new mold may be inserted.

It is therefore an object of this invention to provide an improved method for casting viscous material in a mold.

Another object of this invention is to provide an improved apparatus whereby a single charge of feed-stock highly viscous material may be used to cast one or more molds in succession.

Still another object of this invention is to provide an improved apparatus whereby a single charge of feed-stock highly viscous material may be used to cast more than one mold simultaneously.

A further object of this invention is to provide an improved casting apparatus whereby filled molds may be readily replaced by empty molds.

Still a further object of this invention is to provide an improved apparatus whereby feed-stock of highly viscous material containing components such as metal staples may be forced through a casting plate having patterned holes so that the components contained in the viscous material will be in planar alignment within the filled mold.

These and other objects of this invention will be apparent from the detailed description which follows, when taken together with the appended drawing.

BRIEF DESCRIPTION OF THE DRAWING The appended drawing is a cutaway view of the pressure casting apparatus of the present invention during the casting process.

DESCRIPTION OF THE PREFERRED EMBODIMENT The instant invention concerns an improved casting apparatus. The feed-stock for this apparatus is a highly viscous material capable of flowing. The viscous material may, for example, be a polymer which is to be cast and cured. The polymer is mixed in the normal manner and may have additives therein, for instance, metal staples. Normally, the additive components contained in the viscous material will then have random dispersion and orientation throughout the mixture. However, by extruding the viscous material through a casting plate a planar orientation of the additive components will be attained.

Reference is made to the drawing showing the instant invention during the casting process. A casting mold 10 is removably mounted to a base plate 12 by a fastening means 14, which may be a bolt and nut arrangement, or may be a bolt screwed into the base plate 12. Legs 16 are rigidly affixed to the base plate 12. The legs 16 may serve several functions; for example, they provide clearance be- 3 tween a floor and the base plate 12. Also, the legs 16 may be positioned to slide along rails or to engage in a conveyor system to facilitate handling.

A cylindrical vacuum bell 18 has outwardly extending flanges 20 and 22 at each end thereof. One end of the vacuum bell 18 rests on the base plate 12 and surrounds the casting mold 19. In order to properly assure alignment of the vacuum bell 18 on the base plate 12, index pins 24 may be rigidly mounted on the base plate 12 and positioned to fit into index slots 26 located in flange 20. An air tight sealing means between the vacuum bell 18 and the base plate 12 may be attained by an O-ring 28 located between the base plate 12 and the flange 20. The O-ring 28 may be partially imbedded in a peripheral slot 30 in order to assure proper location of the Oring 28.

Resting on the upper end of the vacuum bell 18 is a pressure casting head 32. There is an O-ring 34 located between the flange 22 and the pressure casting head 32 for the purpose of insuring an air tight seal between the two components.

The pressure casting head 32 is fabricated with a ring plate 36 as a base. A cylindrical pressure casting spacer member 38 separates the ring plate 36 from a head plate 40. O-rings 42 and 44 may be located around the edges of the spacer member 38 in order to insure an air tight seal. The head plate 40 has an inlet opening 46 in substantially the center thereof. The headplate 40 is spacially separated from but rigidly connected to the ring plate 36 by a connecting means, as for example, by head bolts 41 or machine screws. In order that the casting head 32 may be properly positioned on the vacuum bell 18, there may be index pins between the flange 22 and the ring plate 36.

The ring plate 36 has an opening therein, which opening has a diameter of substantially the same diameter as that of the interior of the vacuum bell 18. Overlapping and resting on top of the ring plate 36 is a casting plate 48 which covers the opening in the ring plate 36. Casting plate 48 has pattern holes 50 through it so that viscous material may be forced through the patterned holes 50 from the casting head 32 and be deposited in the mold 10. As a matter of convenience, the casting plate 48 may have two or more casting plate handles 52 rigidly attached thereto. The casting plate 48 may also have index pins attached to it to fit into index holes located in the ring plate 36 for the purpose of aiding the positioning of the casting plate 48 onto the ring plate 36. The holes 50 may be of uniform diameter or they may vary in size. Whether or not the holes 50 vary in size will be a function of the ability of the viscous material to be forced therethrough at substantially equal rates so that the buildup in mold will be substantially uniform. In the event the holes 50 do vary in diameter, the smaller holes would be located adjacent the center of the casting plate 48 and the holes 50 would increase in size as they are located further from the center.

It is readily apparent that the casting head 32 may be rigidly connected to the vacuum bell 18 and the vacuum bell 18 may be in turn rigidly connected to the base 12. It was found in practice that the mentioned rigid connections were not required. The O-rings 28 and 34 provided a sufficient sealing means between the components such that when the vacuum was drawn on the vacuum bell 18 the separate components were drawn together, provided suflicient connection so that the component parts would not separate from each other, and were air-tight.

A means for drawing a vacuum within the vacuum bell 18 is connected thereto. One means of drawing a vacuum on the vacuum bell 18 is to connect a vacuum pump 54 to the vacuum bell 18 by use of a vacuum connection nipple 56 being operably connected to the vacuum bell 18.

In order that the viscous material may be fed into the casting head 32, a transfer pot S8 is operably connected thereto. One connecting means between the transfer pot 58 and the casting head 32 may be made up of a pipeline arrangement 60. The pipeline arrangement 60 has a piping flange 62 rigidly attached to the head plate 40 and covering the inlet opening 46 in such a manner that viscous material may pass through the piping flange 62 through the inlet opening 46 into the casting head 32. A bellows pipe 64 provides a flexure means and connects the piping flange 62 to a control valve 66. The control valve 66 as a flow control means is connected to the discharge of the transfer pot 58 by a pipe 68.

The transfer pot 58 receives a charge of viscous material to be cast. The transfer pot 58 is then sealed and pressurized. One means of pressurizing the transfer pot S8 is to connect a pressure pump 70 to the transfer pot 58 through a pressurizing nipple 72. As any viscous material may be cast by use of this invention, the pressurizing gas must be compatible with the viscous material used as feedstock. It is readily apparent that air may be used as the pressurizing gas. However, in some instances where polymers are being cast, it may be desirable to use gaseous nitrogen as the pressurizing media.

Though disclosed herein as pressure casting one or more molds in succession, it is readily apparent that this invention may be used to pressure cast one or more molds simultaneously. A single transfer pot 58 may be connected to one or more control valves 66 by proper manifold arrangement well known to those with ordinary skills in the art.

During the casting operation, fascicles 74 are being extruded through the pattern holes 50. The casting head 32 is of sufficient height above the mold 10 that the extruded fascicles 74 stretch and break due to their own weight prior to being deposited into the mold 10. The fascicles 74 are elongated when they fall into the mold 10 so that they come to rest in a horizontal plane. Any components contained in the fascicles 74, for example, metal staples, will be oriented in the horizontal plane; however, they may be randomly dispersed within that horizontal plane. The fascicles 74 being a viscous material and in a vacuum environment so that voids will not be present between the fascicles 74, they tend to flow together into a homogeneous mass. Alternatively, the length of the fascicles may be controlled by vibrating the pressure casting apparatus by means well known to those having ordinary skill in the art.

The rate at which the mold 10 is filled may be controlled and timed so that it can be determined when one casting operation is complete. Rather than relying on time, it is preferable that the casting operation be capable of observation. A transparent viewing port 76 provides a viewing means through the vacuum bell 18 so that the mold 10 may be observed during the casting operation.

In operation, the transfer pot 58 is loaded with a feedstock charge 78 of highly viscous material. The transfer pot 58 is closed so it can be pressurized. A vacuum of approximately 27 inches of mercury is drawn on the vac uum bell 18 by the vacuum pump 54. The transfer pot is pressurized by the pressure pump 70 to a predetermined pressure. The control valve 66 is opened to allow the flow of the highly viscous material feedstock charge 78. The pressure causes the material to flow from the transfer pot 58, through the pipeline arrangement 60 into the pressure casting head 32. The differential pressure between the vacuum bell 18 and the transfer pot 58 causes the highly viscous material to be extruded through the patterned outlet holes 50. As the casting plate 48 is of sufficient height above the mold 10, the fascicles 74 stretch and break of their own weight before being dropped into the mold 10. The stretched or elongated fascicles drop into the mold 10 through a vacuum environment and fall into a horizontal plane or arrangement. The environment within the bell 18 being a vacuum, no voids are allowed to form between the fascicles. The highly viscous material will flow together and become a homogeneous mass within the mold 10.

When the mold is filled, the casting operation is terminated. The vacuum within the bell 18 is released so that the bell 18 returns to ambient pressure. The pressure on the transfer pot 58 is reduced to ambient pressure and the control valve 66 is closed. Alternatively, the valve 66 may be closed and it is not essential to reduce the pressure on the transfer pot 58. The pressure casting head 32 is kept full of the highly viscous material for the next casting operation. The flexure means or bellowed pipe 64 allows the casting head 32 to be lifted from the vacuum bell 18 so that together the base plate 12 and vacuum bell 18 may be removed from under the casting head 32 and new ones placed in position. The casting operation can then be repeated while the filled mold 10 is further processed for curing the cast material.

The casting process follows a programmed procedure in order to obtain the best cast. The transfer pot 58 is charged with a viscous material for casting. A vacuum of approximately 27 inches of mercury is drawn upon the vacuum bell 18 before the transfer pot 58 is pressurized. The transfer pot 58 is then pressurized to fill the casting head 32 and force the viscous material through the casting plate 48. The pressure applied to the transfer pot 58 will determine the rate of cast and the sizing of the fascicles 74, as the casting plate 48 is high enough above the mold 10 so that the extruded material stretches and breaks into fascicles 74 due to gravitational forces. Therefore, the pressure on the transfer pot 58 is adjusted to provide the rate of cast and size of fascicles 74 desired. When the mold 10 becomes filled to the desired depth, the casting process is terminated by releasing the vacuum on the vacuum bell 18. The control valve 66 is closed so that the casting head 42 will be maintained full and thereby prevent the development of voids. The casting head 32 may then be raised from the vacuum bell 18 so that the vacuum bell 18 and base plate 12 may be removed and a fresh one inserted.

What is claimed is:

1. The method of pressure casting highly viscous material into a mold, the mold being contained within a vacuum bell having a pressure casting head mounted thereon, the method comprising the steps of:

creating a pressure differential between the pressure being exerted on the material in the casting head and the pressure within the vacuum bell by at least drawing a vacuum on said bell;

pressure feeding, by reason of said pressure differential,

a highly viscous material through a casting plate mounted in said casting head, said plate having patterned holes therein which said plate is at a distance above the mold so that the extruded viscous material stretch and break into elongated fascicles due to gravitational forces; and

gravity depositing said fascicles into said mold whereby said fascicles fill the mold by a substantially uniform build-up and the fascicles come to rest horizontally in the mold so that solid particles, such as metal staples, contained in said material are oriented in a horizontal plane.

2. The method of pressure casting highly viscous material into a mold as described in claim 1, wherein said mold is maintained in a vacuum environment during the casting operation whereby the material deposited in the mold will become a homogeneous mass.

3. The method of pressure casting highly viscous material into a mold, the mold being contained within a vacuum bell having a pressure casting head mounted thereon, the method comprising the steps of:

creating a vacuum within a vacuum bell containing the mold;

pressurizing a transfer pot containing the viscous material to be cast;

forcing the viscous material from said transfer pot into a pressure casting head, said transfer pot being operably connected to said casting head;

extruded the viscous material into fascicles from said pressure casting head through a casting plate having holes therein, said plate being mounted in said casting head; and

depositing the extruded material into said mold whereby said fascicles fill the mold by a substantially uniform buildup and the fascicles come to rest horizontally in the mold so that solid particles, such as metal staples, contained in said material are oriented in a horizontal plane and the material deposited in the mold will become a homogeneous mass.

4. The method of pressure casting highly viscous material into a mold as described in claim 3, and in addition, the steps of:

reducing said vacuum to ambient pressure;

releasing said pressure on said transfer pot;

removing said material and said mold; and

curing said material.

5. The method of pressure casting highl viscous material into a mold as described in claim 3, and after extruding the viscous material, the additional step of:

dropping, by reason of gravitational forces, the viscous material through a vacuum environment.

References Cited UNITED STATES PATENTS 3,311,678 3/1967 Brock et al. 264-3 3,255,281 6/1966 Alexander 264-3 3,261,255 7/1966 Glick et a1 264-3X 3,379,796 4/1968 Hewson et a1. 2643 BENJAMIN R. PADGETT, Primary Examiner S. J. LECHERT, JR., Assistant Examiner US. Cl. X.R. 

